In the current grant cycle, we identified null mutations in a novel gene, Lpin1 (lipin), as the cause of lipodystrophy in the fatty liver dystrophy (fld) mouse. The fld mouse is a model of generalized lipodystrophy, with a lack of normal adipose tissue, insulin resistance, and susceptibility to atherosclerosis. We have established that lipin is abundantly expressed in adipose tissue and skeletal muscle. In the adipocyte, lipin is induced at two distinct time points-first, shortly after initiation of differentiation, acting upstream of the PPARgamma transcriptional regulator, and later, in mature adipocytes. The requirement for lipin in differentiation accounts for the lipodystrophy observed in lipin-deficient mice. Furthermore, transgenic mice with enhanced lipin expression specifically in mature adipocytes become obese, suggesting a specific role for lipin after differentiation. Lipin also has an important role in muscle. Using both lipin-deficient and lipin transgenic mice, we have demonstrated that lipin expression levels in muscle are a determinant of energy metabolism, with null expression causing increased energy expenditure, and enhanced expression causing reduced energy expenditure, leading to obesity. Thus, lipin is unique among known factors in its ability to cause the absolute extremes of adiposity-ranging from lipodystrophy to obesity-depending on its expression levels in peripheral tissues. The proposed studies seek to further elucidate the cellular and molecular functions of lipin in adipose tissue and muscle biology. These studies will also provide the first evaluation of genetic variation in human lipin gene structure and expression and its potential association with features of the metabolic syndrome. Using a combination of genetic, molecular, and biochemical approaches, we will address the following questions: (1) What is the role of lipin in differentiating and mature adipocytes?; (2) How do lipin levels in muscle modulate energy expenditure?; (3) What is the identity of lipin-protein interacting proteins?; and (4) Are variations in human lipin gene sequence or expression levels associated with clinical aspects of the metabolic syndrome?